5G communication systems use massive multiple input multiple output (MIMO) technology and beamforming. Massive MIMO technology uses a very high number of antennas. Massive MIMO technology provides increased data streams, small scale fading elimination and larger beamforming gain.
FIG. 1 illustrates an example of a massive MIMO system which uses beamforming. In FIG. 1, a next Generation Node B (gNB) transmits signals to several user equipment (UE). The gNB comprises a very high number of antennas. The antennas of the gNB are grouped into several sub-arrays. Additionally, the gNB uses beamforming to transmit signals to the UEs. Similarly, the gNB receives signals from the UEs through the beams. The UEs access a network through the gNB.
FIG. 2 illustrates an example of a transceiver of a massive MIMO system which uses all digital beamforming. In FIG. 2, the transceiver inputs Ns baseband signals to a baseband precoding block FBB. FBB outputs Lt precoded signals. The precoded signals are the inputs of Lt digital-to-analog converters (DAC). The Lt outputs of the DACs are the inputs of Lt radio frequency (RF) chains. The Lt outputs of the RF chains are transmitted through the Nt antennas of the transceiver. Similar to FIG. 1, the transceiver of FIG. 2 also uses beamforming. FBB performs precoding in order to transmit the precoded signals in different beams.
All digital beamforming is limited by: space, signal processing complexity and cost, including high power consumption.
FIG. 3 illustrates an example of a transceiver of a massive MIMO system which uses hybrid digital/analog beamforming. Hybrid beamforming performs precoding in the digital domain and in the analog domain. Similar to FIG. 2, the transceiver inputs Ns baseband signals to a baseband precoding block FBB. FBB performs precoding in digital domain. FBB outputs Lt precoded signals. The precoded signals are the inputs of Lt DACs. The Lt outputs of the DACs are the inputs of Lt RF chains. However, different from FIG. 2, in FIG. 3, the Lt outputs of the RF chains are the inputs of a RF precoding block FRF. FRF performs precoding in the analog domain. FRF outputs Nt precoded signals, which are transmitted through the Nt antennas of the transceiver.
Comparing FIGS. 2-3, the number of baseband signals Ns is less than or equal to the number of RF chains Lt: Ns≤Lt. In FIG. 2, the number of RF chains Lt equals the number of antennas Nt: Lt=Nt. However, in FIG. 3, since hybrid beamforming has RF precoding, the number of RF chains Lt may be less than the number of antennas Nt: Lt<Nt.
FIGS. 4A, 4B and 4C illustrate examples of transceivers which use hybrid beamforming. In FIGS. 4A, 4B and 4C, the transceiver inputs Ns baseband signals to a digital precoder FB. FB outputs NR precoded signals. The precoded signals are the inputs of NR RF chains. The RF chains perform frequency upconversion and outputs a plurality of RF signals. The analog precoder FR performs analog precoding on the plurality of RF signals. After analog precoding, a plurality of power amplifiers (PA) amplify the plurality of RF signals. Lastly, after amplifying, the antenna sub-arrays of the transceiver transmit the plurality of RF signals.
FIG. 4A shows a transceiver with a fully-connected structure, where each RF chain is connected to all antennas. In FIG. 4A, the 1st RF signal of the 1st RF chain is connected to the 1st antenna, the 2nd RF signal of the 1st RF chain is connected to the 2nd antenna, and the Nt-th RF signal of the 1st RF chain is connected to the Nt-th antenna. Similarly, the 1st RF signal of the NR-th RF chain is connected to the 1st antenna, the 2nd RF signal of the NR-th RF chain is connected to the 2nd antenna, and the Nt-th RF signal of the NR-th RF chain is connected to the Nt-th antenna. In FIG. 4A, each RF is connected to all antenna sub-arrays.
FIG. 4B shows a transceiver with a partially-connected structure, where each sub-array is connected to only a single RF chain. In FIG. 4B, the 1st RF signal of the 1st RF chain is connected to the 1st antenna of the 1st antenna sub-array, the 2nd RF signal of the 1st RF chain is connected to the 2nd antenna of the 1st antenna sub-array, and the N-th RF signal of the 1st RF chain is connected to the N-th antenna of the 1st antenna sub-array. Similarly, the 1st RF signal of the NR-th RF chain is connected to the 1st antenna of the NR-th antenna sub-array, the 2nd RF signal of the NR-th RF chain is connected to the 2nd antenna of the NR-th antenna sub-array, and the N-th RF signal of the NR-th RF chain is connected to the N-th antenna of the NR-th antenna sub-array.
FIG. 4C shows a transceiver with a hybridly-connected structure, where each antenna sub-array is connected to multiple RF chains. In FIG. 4C, each sub-array RF chain comprises S RF chains. After analog precoding, each antenna of the antenna sub-array transmits RF signals from S RF chains.
FIGS. 5A and 5B illustrate another example of transceivers which use hybrid beamforming. In FIG. 5A, the transceiver inputs Ns baseband signals to a digital precoder FB. FB outputs NR precoded signals. The precoded signals are the inputs of NR DACs. After the precoded signals are converted to analog signals by the DACs, the RF chains perform frequency upconversion and outputs a plurality of RF signals. The analog precoder FR performs analog precoding on the plurality of RF signals. After analog precoding, a plurality of power amplifiers (PA) amplify the plurality of RF signals. Lastly, after amplifying, the antenna sub-arrays of the transceiver transmit the plurality of RF signals. Additionally, in FIG. 5A, analog precoding is performed through analog weighting, where the RF signals are multiplied by a weighting factor.
FIG. 5B shows several components of FIG. 5A. Phase shifters are analog electronic circuits which perform analog precoding. The input of the phase shifter is an analog signal, and outputs the analog signal with a predetermined phase shift. Phase shifters may include a DAC to convert a digital control signal input by a digital circuit to an analog signal, to control the phase shifter. Power amplifiers are analog electronic circuits which increase the power of the input analog signal. Antenna elements are antennas. A group of antenna elements may be connected together to form an antenna array. The antenna array may work as a single antenna to transmit and receive radio waves.
FIGS. 6A and 6B illustrate examples of antennas of 4G communication systems and 5G communication systems. FIGS. 6A and 6B show a base station providing network access to several user terminals in a coverage area.
FIG. 6A illustrate an example of antennas for 4G communication systems. In FIG. 6A, the antenna provides network access to several user terminals. Some user terminals are outside the coverage area of the antenna and are not connected to the network.
FIG. 6B illustrate an example of an antenna array for 5G communication systems. In FIG. 6B, the base station uses a massive active phased antenna array (APPA) and beamforming. Thus, the coverage area of the base station is divided into beams. A user terminal accesses the network through one of the beams. Since the base station uses beamforming, the base station can direct the power of the signal towards the user and provide network access to the user, even if the user is not near the base station.
FIG. 7 illustrates a block diagram of ideal antenna calibration. Antenna calibration comprises estimation and compensation. The transceiver may receive an input of a signal xn to be transmitted. Block hn groups the channel effects caused by the transceiver hardware on signal xn. First, the antenna calibrator estimates the channel effects hn. Then, the antenna calibrator performs compensation of the channel effects by pre-filtering the input signal xn with filter response (1/hn). After performing estimation and compensation, the antenna transmits signal xn. Compensation may be provided by pre-filtering the input signal xn with a filter of response (1/hn) in time domain, or by multiplying a filter with frequency response (1/H) in frequency domain, where H is the frequency response of hn.
FIGS. 8A and 8B illustrate examples of impulse responses caused by hardware impairments and compensation by antenna calibration. In FIGS. 8A and 8B, the transceiver comprises N antenna elements. Antenna calibration performs compensation for each antenna.
FIG. 8A illustrates an example of impulse responses with one impulse only. In FIG. 8A, the impulse response at antenna element #1 is one impulse h1. The impulse response at antenna element #2 is one impulse h2. The impulse response at antenna element #N is one impulse hN. In ideal calibration, there is no error in estimation, and the calibrator performs compensation by multiplying the signal to be transmitted by the inverse of the impulse response. Thus, after compensation, the impulse responses at antenna element #1, antenna element #2 and antenna element #N have one impulse of value 1.
FIG. 8B illustrates an example of impulse responses with several impulses. In FIG. 8B, the impulse response at antenna element #1 has impulses h11, h12, h13, . . . , h1p. The impulse response at antenna element #2 has impulses h21, h22, h23, . . . , h2p. The impulse response at antenna element #N has impulses hN1, hN2, hN3, . . . , hNp. In ideal calibration, there is no error in estimation. The calibrator performs compensation with an infinite impulse response (IIR) filter, where the frequency response of the IIR filter is the inverse of the frequency response at the corresponding antenna element. Thus, after compensation, the impulse responses at antenna element #1, antenna element #2 and antenna element #N have one impulse of value 1.
Due to the benefits described above, employing antenna calibration in a hybrid beamforming system is desired. However, antenna calibration in a massive MIMO system also presents particular challenges. A massive MIMO system comprises a very high number of antenna elements, and each antenna element requires a feedback circuit. Since the number of antenna elements is very high, it is desirable that antenna calibration is performed for a feedback signal which combines the transmit signals of several antenna elements. In antenna calibration for conventional systems, a combined feedback signal is not needed since the number of antenna elements is not high.
The disclosure is directed to a transceiver with many antenna arrays using hybrid beamforming and antenna calibration. The transceiver of the disclosure may comprise one single feedback circuit that may combine the transmit signals of the antenna elements into one feedback signal. The calibrator is able to recover the transmit signals of the antenna elements because the calibration method uses orthogonal scrambling sequences. Thus, the transceiver of the disclosure may perform antenna calibration while reducing feedback circuit hardware complexity and cost.